Robust Bifunctional Core−Shell MOF@POP Catalyst for One-PotTandem ReactionMei-Hong Qi, Ming-Liang Gao, Lin Liu, and Zheng-Bo Han*

College of Chemistry, Liaoning University, Shenyang 110036, P. R. China

*S Supporting Information

ABSTRACT: A new bifunctional acid−base catalyst,core−shell UiO-66@SNW-1, with robust chemical andthermal stability, recyclability, and durable catalyticactivity is synthesized by a convenient, universal strategy.Interestingly, this hybrid material can effectively catalyzedeacetalization-Knoevenagel condensation reaction in thepresence of excellent compartmentalization to spatiallyisolate opposing acid−base sites.

One-pot tandem reactions have attracted great attentionowing to their natures of simple synthetic procedures

and atom efficiency.1 The multifunctional solid catalystspossessing different catalytic sites are extremely applicable tocatalyze tandem reactions.2 For example, metal−organicframeworks (MOFs), montmorillonite, porous organic poly-mers (POPs), mesoporous silicas, etc., which containcoexistent acidic and basic sites, have been applied for tandemreaction.3

MOFs, a promising class of porous crystalline materials,assemble with metal centers and various organic linkers.4

Because of their large surface areas, tunable pore size,recoverability, and chemical diversity, MOFs usually showgreat potential for versatile applications, such as gas storageand separation, carbon capture, sensing, drug delivery,catalysis,5 etc.6 Recently, the catalytic property of MOFs hasattracted a great attention by researchers. Most MOFs areappropriate for single-site catalysis, thus preparing multifunc-tional MOFs catalyst has remained challenging.7 The mainlimitation in the catalytic field depended on how to introduceother functional sites and maintaining the natural features.8

MOF-based hybrid materials are an emerging multifunctionalmaterial that combines MOFs with other MOFs, enzymes,magnetic metal oxides, and polymers.9 These hybrid materialsdemonstrate admirable properties compared with singlespecies in a catalytic field. Porous organic polymers (POPs),as a new kind of developed porous material, are entirelyconstructed from organic building blocks with reticularchemistry through covalent bond formation.10 POPs possesshigh surface areas, variable functional groups, and adjustablepore sizes that offer unprecedented possibilities for catalysis.11

However, to prepare acid−base bifunctional catalysts of POPsis not straightforward, especially when Lewis acid and basesites tend to quench each other. Therefore, to develop newtypes of hybrid MOFs@POPs is of great strategic significanceowing to their potentially improved performance over those ofindividual components and extensive applications in heteroge-

neous catalysis. Recently, Zhang and co-workers successfullydeveloped a new type of NH2-MIL-68@TPA-COF core−shellhybrid material and applied it to degradation of rhodamineB.12 Xie et al. reported a MOF@POP nanocomposite (UNM)that showed great potential in biomedical fields and cancertreatment.13 The Hu group also developed a series of MIL-101@Pt@FeP-CMP composite materials that possess Lewisacid sites to activate CO bond.14 To the best of ourknowledge, the performances of these core−shell MOFs@POPs hybrid materials are superior to the individualcomponents.On the basis of the above-mentioned strategies, the use of

MOFs@POPs hybrid materials as a single catalytic systemcould combine their respective advantages while effectivelyeliminating their weakness. Therefore, the fabrication ofMOFs@POP hybrid materials with multifunctional catalyticsites would be of great promise for extending the application ofMOFs and POPs. In this work, a new MOF@POP (UiO-66@SNW-1) was prepared under solvothermal conditions andpresented as an original platform for catalyzing the one-pottandem deacetalization-Knoevenagel condensation reaction(Scheme 1). Delightedly, the core−shell UiO-66@SNW-1,

consisting of Lewis acid sites (the Zr clusters in UiO-66) andBrønsted base sites (aminal groups in SNW-1), can act as ahighly efficient bifunctional catalyst for the one-pot tandemreaction. Impressively, the bifunctional core−shell UiO-66@SNW-1 catalyst exhibited more preeminent catalytic perform-ance than the the parent UiO-66, SNW-1, and UiO-66-NH2.To investigate the crystallinity and structural stability of

UiO-66@SNW-1 hybrid material, the comprehensive charac-

Received: August 15, 2018

Scheme 1. Schematic Illustration Showing the Fabricationof UiO-66@SNW-1

Communication

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© XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.8b02303Inorg. Chem. XXXX, XXX, XXX−XXX

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terization was performed by powder X-ray diffraction (PXRD).As shown in Figure S1, the identically strong Bragg diffractionpeaks of UiO-66 and UiO-66@SNW-1 revealed that SNW-1 isamorphous and the crystalline structure of UiO-66 does notalter in the process of growth of POPs. The FT-IR spectrum ofUiO-66@SNW-1 matched well with UiO-66 as well as SNW-1and further indicated that the UiO-66@SNW-1 was success-fully formed. The presence of the distinct stretching band forthe triazine ring at ≈1550 and 1480 cm−1 indicates theformation of SNW-1 (Figure S2 and S3). In addition, thechemical and thermal stability of core−shell UiO-66@SNW-1hybrid material were investigated by PXRD. When the hybridmaterial was soaked in different solvents for 48 h, PXRDpatterns remained unchanged, which indicated no frameworkcollapse during chemical stability tests (Figure 1 and S4). FT-

IR spectrum further proved that the structure is intact withoutdamage (Figure S5). The variable-temperature PXRD andthermogravimetric analysis (TGA) were also investigated, andthe results revealed that UiO-66@SNW-1 hybrid materialcould be stable up to 400 °C in air (Figure S6 and S7).The core−shell UiO-66@SNW-1 hybrid material was

investigated by scanning and transmission electron microscopy(SEM and TEM, in Figure 2). As shown in Figure 2a−c, SEMimages showed that UiO-66 has an angulated nanocrystal withsmooth surfaces and the surfaces of the pristine UiO-66become rough after growing SNW-1. The SNW-1 coating inthe TEM images can be distinguished by a lighter contrast thanthat of the inner MOF. TEM image analysis of UiO-66@SNW-1 revealed that the surface of the UiO-66 is coated with SNW-1 material (∼25 nm thickness) (Figure 2d). The weight ratioof UiO-66:SNW-1 in UiO-66@SNW-1 is determined to be≈1:0.56 on the basis of the TGA analyses. The N2 sorptionproperties of core−shell UiO-66@SNW-1 material wereanalyzed by the Brunauer−Emmett−Teller (BET) method at77 K. The results of core−shell UiO-66@SNW-1 hybridmaterial exhibited type IV isotherms, and the surface area wasmeasured as 354 m2 g−1. The N2 sorption properties of theUiO-66@SNW-1 revealed that the microporous characteristicsof the hybrid material with a very high degree of cross-linkingby SNW-1 (Figure S8).Recently, the development of bifunctional heterogeneous

catalysts for promoting one-pot tandem reactions has currently

drawn much attention. The UiO-66@SNW-1 hybrid materialcontained abundant unsaturated Zr centers, which can be usedas Lewis acid sites for catalyzing deacetalization reaction andthe aminal groups in SNW-1 as a weak Brønsted base sites canbe utilized for catalyzing Knoevenagel reaction. Thus,deacetalization-Knoevenagel reaction was selected to explorethe catalytic performance of core−shell UiO-66@SNW-1hybrid material. The catalytic activity of hybrid material wasevaluated in the presence of 50 mg of activated UiO-66@SNW-1 as a model reaction, wherein benzaldehydedimethyla-cetal (a) (2.0 mmol) reacts with malononitrile (2.1 mmol) inDMSO-d6 (2.0 mL) under a N2 atmosphere at 80 °C for 12 h.The conversion and yield were monitored by 1H NMRspectroscopy. In this case, excellent yield (99.6%) of thedesired product 2-benzylidenemalononitrile (c) was observed(Table 1). Compared with other previous catalysts such as Yb-BCD-NH2, MIL-101(Al)-NH2, and PCN-124 (Table S2),15

UiO-66@SNW-1 hybrid material also exhibited good catalytic

Figure 1. Chemical stability tests for UiO-66@SNW-1 monitored byPXRD.

Figure 2. SEM images of (a) UiO-66, (b) SNW-1, (c) UiO-66@SNW-1 hybrid material. (d) TEM image of UiO-66@SNW-1 hybridmaterial.

Table 1. One-Pot Tandem Deacetalization-KnoevenagelCondensation Reaction and Monitored by 1H NMRa

entry conv of a (%) yield of b (%) yield of c (%)

UiO-66@SNW-1 99.6 trace 99.6second cycle 99.5 trace 99.5third cycle 99.4 trace 99.4fourth cycle 99.6 trace 99.6fifth cycle 99.5 trace 99.5UiO-66-NH2 75.2 0.9 74.3UiO-66 99.5 83.3 16.2SNW-1 ∼0 trace traceno catalyst ∼0 trace traceUiO-66+SNW-1 99.4 17.5 81.9

aReaction conditions: benzaldehyde dimethylacetal (2.0 mmol),malononitrile (2.1 mmol), DMSO-d6 (2 mL), and catalyst (50 mg);reaction temperature, 80 °C; reaction time, 12 h.

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activity for deacetalization-Knoevenagel reaction. In detail, thekinetic investigation revealed that the reaction completeswithin 12 h (Figure S9). On the basis of the controlexperiments, the tandem reaction can be considered as twosequential steps: first, the unsaturated Zr clusters catalyzedbenzaldehyde dimethylacetal (a) to generate benzaldehyde(b); second, the aminal groups in SNW-1 catalyzedKnoevenagel condensation reaction to produce 2-benzylide-nemalononitrile (c). To better comprehend the necessity ofthe catalyst for this tandem reaction, the same model reactionwas carried out without catalyst. The reaction without UiO-66@SNW-1 can scarcely react to product over 12 h. WhenUiO-66 used as catalyst to boost this tandem reaction, the firststep deacetalization reaction was efficient but the second stepKnoevenagel condensation reaction did not take place. Whenonly SNW-1 was used as catalyst, the target product could notbe detected. In addition, as contrasted with UiO-66-NH2, thecatalytic effect of UiO-66@SNW-1 was better on deacetaliza-tion-Knoevenagel condensation reaction. The activity on thephysical mixture of UiO-66 and SNW-1 (UiO-66: SNW-1 =1:0.56) was relatively lower than UiO-66@SNW-1 whichconfirmed the advantages of the core−shell structural design ofthe UiO-66@SNW-1 material. The reason was assigned to thetwo catalytic parts being joined together, making the masstransfer process more efficient.16 A leaching test indicated thatno active species leached into the solution and theheterogeneity of the catalyst is directly proved (Figures S10).After tandem reaction, the catalyst could be removed bycentrifuging and reused at least for five cycles without anoticeable change in its activity (Figures S11). The crystallinityand structural integrity of core−shell UiO-66@SNW-1 hybridmaterial did not change even after five cycles, indicating itsgreat recyclability and excellent stability for the one-pottandem deacetalization-Knoevenagel condensation reaction(Figures S12, S13, S14).In conclusion, a bifunctional acid−base catalyst, core−shell

UiO-66@SNW-1 hybrid material, has been successfullydeveloped to boost the catalytic performance toward one-pottandem deacetalization-Knoevenagel condensation reaction.The core−shell UiO-66@SNW-1 hybrid material containedboth Lewis acid and Brønsted base sites and showed largesurface areas as well as high chemical and thermal stability. Inthe meantime, high chemical stability and thermal stability,microporous characteristics of core−shell UiO-66@SNW-1hybrid material is markedly beneficial for heterogeneouscatalysis. Finally, this work highlighted the prominent catalyticperformance of core−shell UiO-66@SNW-1 hybrid materialfor one-pot tandem deacetalization-Knoevenagel condensationreaction. This strategy would be extensively employed in thedevelopment of functional MOFs hybrid materials and highperformance heterogeneous catalyst. Our ongoing work willdeal with systematically preparing MOFs@POPs hybridmaterials that can be applied in tandem reaction.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.inorg-chem.8b02303.

Materials and general methods, PXRD pattern, FT-IRspectra, TGA curves, adsorption−desorption isotherms,1H NMR spectra, conversions and yields from hot

filtration reactions, recycle capacity tests, SEM and TEMimages, and data from condensation reactions (PDF)

■ AUTHOR INFORMATIONCorresponding Author*Z.-B. Han. E-mail: ceshzb@lnu.edu.cn.ORCIDZheng-Bo Han: 0000-0001-8635-9783NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was granted financial support from the NationalNatural Science Foundation of China (Grant 21671090 and21701076).

■ REFERENCES(1) (a) Dhakshinamoorthy, A.; Garcia, H. Cascade reactionscatalyzed by metal organic frameworks. ChemSusChem 2014, 7,2392−2410. (b) Kim, J. H.; Ko, Y. O.; Bouffard, J.; Lee, S. G.Advances in tandem reactions with organozinc reagents. Chem. Soc.Rev. 2015, 44, 2489−2507.(2) (a) He, Y.; Li, B.; O’Keeffe, M.; Chen, B. Multifunctional metal-organic frameworks constructed from meta-benzenedicarboxylateunits. Chem. Soc. Rev. 2014, 43, 5618−5656. (b) Dhakshinamoorthy,A.; Asiri, A. M.; Garcia, H. Mixed-metal or mixed-linker metal organicframeworks as heterogeneous catalysts. Catal. Sci. Technol. 2016, 6,5238−5261.(3) (a) Beyzavi, M. H.; Vermeulen, N. A.; Howarth, A. J.;Tussupbayev, S.; League, A. B.; Schweitzer, N. M.; Gallagher, J. R.;Platero-Prats, A. E.; Hafezi, N.; Sarjeant, A. A.; Miller, J. T.; Chapman,K. W.; Stoddart, J. F.; Cramer, C. J.; Hupp, J. T.; Farha, O. K. AHafnium-Based Metal-Organic Framework as a Nature-InspiredTandem Reaction Catalyst. J. Am. Chem. Soc. 2015, 137, 13624−13631. (b) Li, H.; Pan, Q. Y.; Ma, Y. C.; Guan, X. Y.; Xue, M.; Fang,Q. R.; Yan, Y. S.; Valtchev, V.; Qiu, S. L. Three-Dimensional CovalentOrganic Frameworks with Dual Linkages for Bifunctional CascadeCatalysis. J. Am. Chem. Soc. 2016, 138, 14783−14788. (c) Biradar, A.V.; Patil, V. S.; Chandra, P.; Doke, D. S.; Asefa, T. A trifunctionalmesoporous silica-based, highly active catalyst for one-pot, three-stepcascade reactions. Chem. Commun. 2015, 51, 8496−8499.(4) (a) Zhou, H. C.; Long, J. R.; Yaghi, O. M. Introduction to metal-organic frameworks. Chem. Rev. 2012, 112, 673−674. (b) Howarth, A.J.; Liu, Y. Y.; Li, P.; Li, Z. Y.; Wang, T. C.; Hupp, J. T.; Farha, O. K.Chemical, thermal and mechanical stabilities of metal−organicframeworks. Nat. Rev. Mater. 2016, 1, 15018−15032. (c) Zhou, M.;Ju, Z.; Yuan, D. Q. A new metal−organic framework constructed fromcationic nodes and cationic linkers for highly efficient anion exchange.Chem. Commun. 2018, 54, 2998−3001.(5) (a) Nandasiri, M. I.; Jambovane, S. R.; McGrail, B. P.; Schaef, H.T.; Nune, S. K. Adsorption, separation, and catalytic properties ofdensified metal-organic frameworks. Coord. Chem. Rev. 2016, 311,38−52. (b) Song, X. Z.; Qiao, L.; Sun, K. M.; Tan, Z.; Ma, W.; Kang,X. L.; Sun, F. F.; Huang, T.; Wang, X. F. Triple-shelled ZnO/ZnFe2O4 heterojunctional hollow microspheres derived from PrussianBlue analogue as high-performance acetone sensors. Sens. Actuators, B2018, 256, 374−382. (c) Wu, M. X.; Yang, Y. W. Metal-OrganicFramework (MOF)-Based Drug/Cargo Delivery and CancerTherapy. Adv. Mater. 2017, 29, 1606134. (d) Huang, N.; Yuan, S.;Drake, H.; Yang, X. Y.; Pang, J. D.; Qin, J. S.; Li, J. L.; Zhang, Y. M.;Wang, Q.; Jiang, D. L.; Zhou, H. C. Systematic Engineering of SingleSubstitution in Zirconium Metal-Organic Frameworks toward High-Performance Catalysis. J. Am. Chem. Soc. 2017, 139, 18590−18597.(e) Dhakshinamoorthy, A.; Asiri, A. M.; Garcia, H. Metal OrganicFrameworks as Versatile Hosts of Au Nanoparticles in HeterogeneousCatalysis. ACS Catal. 2017, 7, 2896−2919. (f) Tan, Y.-X.; Yang, X.;

Inorganic Chemistry Communication

DOI: 10.1021/acs.inorgchem.8b02303Inorg. Chem. XXXX, XXX, XXX−XXX

C

Li, B.-B.; Yuan, D. Q. Rational design of a flu-type heterometalliccluster-based Zr-MOF. Chem. Commun. 2016, 52, 13671−13674.(6) (a) Liu, L.; Zhang, X. N.; Han, Z. B.; Gao, M. L.; Cao, X. M.;Wang, S. M. An InIII-based anionic metal−organic framework:sensitization of lanthanide (III) ions and selective absorption andseparation of cationic dyes. J. Mater. Chem. A 2015, 3, 14157−14164.(b) Gao, M. L.; Wei, N.; Han, Z. B. Anionic metal−organicframework for high-efficiency pollutant removal and selective sensingof Fe(III) ions. RSC Adv. 2016, 6, 60940−60944.(7) Rogge, S. M. J.; Bavykina, A.; Hajek, J.; Garcia, H.; OlivosSuarez, A.; Sepulveda Escribano, I. A.; Vimont, A.; Clet, G.; Bazin, P.;Kapteijn, F.; Daturi, M.; Ramos Fernandez, E. V.; Llabres i Xamena,F. X.; Van Speybroeck, V.; Gascon, J. Metal−organic and covalentorganic frameworks as single-site catalysts. Chem. Soc. Rev. 2017, 46,3134−3184.(8) Yuan, S.; Qin, J. S.; Li, J.; Huang, L.; Feng, L.; Fang, Y.; Lollar,C.; Pang, J.; Zhang, L.; Sun, D.; Alsalme, A.; Cagin, T.; Zhou, H. C.Retrosynthesis of multi-component metal−organic frameworks. Nat.Commun. 2018, 9, 808.(9) (a) Li, T.; Sullivan, J. E.; Rosi, N. L. Design and preparation of acore-shell metal-organic framework for selective CO2 capture. J. Am.Chem. Soc. 2013, 135, 9984−9987. (b) Lian, X.; Fang, Y.; Joseph, E.;Wang, Q.; Li, J.; Banerjee, S.; Lollar, C.; Wang, X.; Zhou, H. C.Enzyme-MOF (metal-organic framework) composites. Chem. Soc. Rev.2017, 46, 3386−3401. (c) Ke, F.; Qiu, L. G.; Zhu, J. Fe3O4@MOFcore-shell magnetic microspheres as excellent catalysts for the Claisen-Schmidt condensation reaction. Nanoscale 2014, 6, 1596−1601.(d) Kitao, T.; Zhang, Y.; Kitagawa, S.; Wang, B.; Uemura, T.Hybridization of MOFs and polymers. Chem. Soc. Rev. 2017, 46,3108−3133.(10) Sun, Q.; Dai, Z.; Liu, X.; Sheng, N.; Deng, F.; Meng, X.; Xiao,F. S. Highly Efficient Heterogeneous Hydroformylation over Rh-Metalated Porous Organic Polymers: Synergistic Effect of HighLigand Concentration and Flexible Framework. J. Am. Chem. Soc.2015, 137, 5204−5209.(11) Sun, Q.; Dai, Z.; Meng, X.; Xiao, F. S. Porous polymer catalystswith hierarchical structures. Chem. Soc. Rev. 2015, 44, 6018−6034.(12) Peng, Y. W.; Zhao, M. T.; Chen, B.; Zhang, Z. C.; Huang, Y.;Dai, F. N.; Lai, Z. C.; Cui, X. Y.; Tan, C. L.; Zhang, H. Hybridizationof MOFs and COFs: A New Strategy for Construction of MOF@COF Core-Shell Hybrid Materials. Adv. Mater. 2018, 30, 1705454.(13) Zheng, X. H.; Wang, L.; Pei, Q.; He, S. S.; Liu, S.; Xie, Z. G.Metal−Organic Framework@Porous Organic Polymer Nanocompo-site for Photodynamic Therapy. Chem. Mater. 2017, 29, 2374−2381.(14) Yuan, K.; Song, T. Q.; Wang, D.; Zhang, X.; Gao, X. T.; Zou,Y.; Dong, H. L.; Tang, Z. Y.; Hu, W. P. Effective and SelectiveCatalysts for Cinnamaldehyde Hydrogenation: Hydrophobic Hybridsof Metal-Organic Frameworks, Metal Nanoparticles, and Micro- andMesoporous Polymers. Angew. Chem., Int. Ed. 2018, 57, 5708−5713.(15) (a) Zhang, Y.; Wang, Y. X.; Liu, L.; Wei, N.; Gao, M. L.; Zhao,D.; Han, Z. B. Robust Bifunctional Lanthanide Cluster-based Metal-Organic Frameworks (MOFs) for Tandem Deacetalization Knoeve-nagel Reaction. Inorg. Chem. 2018, 57, 2193−2198. (b) Toyao, T.;Fujiwaki, M.; Yu, H.; Matsuoka, M. Application of an amino-functionalised metal-organic framework: an approach to a one-potacid-base reaction. RSC Adv. 2013, 3, 21582−21587. (c) Park, J.; Li, J.R.; Chen, Y. P.; Yu, J.; Yakovenko, A. A.; Wang, Z. U.; Sun, L. B.;Balbuena, P. B.; Zhou, H. C. A versatile metal-organic framework forcarbon dioxide capture and cooperative catalysis. Chem. Commun.2012, 48, 9995−9997.(16) (a) You, C.; Yu, C.; Yang, X.; Li, Y.; Huo, H.; Wang, Z.; Jiang,Y.; Xu, X.; Lin, K. F. Double-shelled hollow mesoporous silicananospheres as an acid−base bifunctional catalyst for cascadereactions. New J. Chem. 2018, 42, 4095−4101. (b) Wang, Z. T.;Yuan, X. F.; Cheng, Q.; Zhang, T. C.; Luo, J. An efficient andrecyclable acid-base bifunctional core-shell nanocatalyst for the one-pot deacetalization-Knoevenagel tandem reaction. New J. Chem. 2018,42, 11610−11615.

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